Planetary gears of this type are generally known. The planet wheels each have two sets of teeth for permanent meshing with two ring gears. They thus form a so-called Wolfrom coupling gear. The speed ratio of the planet carrier to the ring gear provided for the output is designated the Wolfrom ratio. In the planetary gears known already, the central sun wheel meshes directly with the planet wheels and forms a preliminary ratio with the planet wheels and the stationary ring gear of the two ring gears. The Wolfrom ratio, multiplied by the preliminary ratio, results in the total ratio.
Many tasks for such planetary gears, especially in robot technology, require high ratios with, for example, i=170 or still higher. But it is often desirable that the diameter of the central sun wheel not be too small since, for example, it may be provided with a hollow shaft for accommodating a shaft end of an engine or for conveying robot parts. Besides, it is often desirable to limit the Wolfrom ratio. But it is disadvantageous that moderate values regarding tooth rolling activity, noise, efficiency, heating and inertia result only from a limitation of the speeds of the planet carrier. On the other hand, with a limited Wolfrom ratio it also becomes difficult also to achieve elevated total ratios.
Since the driving central sun wheel is directly engaged with the planet wheels, in the formerly known planetary gear designs, it must be equal to them in module and other toothing data.
In the solutions known already, the obtention of high ratios brings, along with the above mentioned disadvantages, the further disadvantage that in cases having a maximum number of three planetary wheels, a weak torque tightness exists and problems can result regarding the uniformity of the output torque. The small number of sun wheel teeth needed for a sufficiently high total ratio brings, together with the problems associated with the parts of a robot environment, the added disadvantages relating to tooth shape, such as undercut or pointed tips. A further disadvantage is that this requires a relatively high number of teeth on ring gears having relatively small modules, and the torque tightness disadvantages resulting therefrom.
Therefore, the problem on which this invention is based is to obtain high ratios with a good utilization of installation space (torque tightness) while, as far as possible, overcoming the above mentioned disadvantages or at least clearly reducing them. In addition, transmission quality is to be improved, that is, vibrations and angular transmission errors must be minimized. Also, the planetary gear must have favorable production characteristics and require no elevated assembly cost.